There is an ongoing need for component miniaturization in radio wave communication devices. For example, smaller and more efficient components are needed for light-weight, hand-portable cellular telephones, wireless local area networks for linking computer systems within office buildings in a readily reconfigurable fashion, wristwatch- and creditcard-sized paging apparatus and other devices for promoting rapid, efficient and flexible voice and data communication.
Filters are needed for a variety of such communications applications wherein small size, light weight and high performance are simultaneously required. Increasing numbers of products seek to employ fixed spectral resources, often to achieve tasks not previously envisioned. Examples include cellular telephones, inter- and intra-facility computer-computer and/or computer-ancillary equipment linkages as well as a host of other, increasingly complex inter-personal and/or -equipment information sharing requirements. The desire to render increasingly complicated communications modes portable and even hand-held and/or -portable and/or pocket-sized places extreme demands on filtering technology in the context of increasingly crowded radio frequency resources.
Acoustic wave filters must meet stringent performance requirements including (i) being extremely robust, (ii) being readily mass produced, (iii) and sharply increasing the performance to size ratio achievable in the frequency range extending from a few tens of MegaHertz to about several GigaHertz. However, there is a need for low passband insertion loss simultaneously coupled with demand for high shape factor and wide bandwidth which is not easily met by acoustic wave filters.
Hypothetically, one approach to satisfying these needs and demands is to use a substrate with an exceptionally high coupling coefficient. This approach would realize an increased bandwidth and improved shape factor. However, existing substrates are limited in the level of available electromechanical coupling coefficient (k.sup.2).
Another approach is to provide two or more such filters on a single substrate. One realization includes a series-parallel arrangement of resonant elements arranged in a ladder structure, i.e., a structure comprising cascaded sections each including a series resonant element followed by a shunt resonant element. Typically, within each section, the antiresonant frequency of the shunt element is chosen to be the resonant frequency of the accompanying series element, providing pure real input and output impedances. The disadvantage of this approach includes a fixed bandwidth for the coupling coefficient (k.sup.2) associated with the chosen substrate material.
Prior art ladder filter structures employing piezoelectric series and/or shunt elements have generally been confined to extremely narrow bandwidths, on the order of 0.01% to 0.1%, in part because they have been implemented with piezoelectric materials having very low electromechanical coupling coefficients. Generally, conventional approaches are such that when three of: the filter material, the impedance, the selectivity and the bandwidth are specified, the fourth is also determined.
What is needed is a ladder filter configuration/ design methodology providing flexible bandwidth, suitable out-of-band rejection and low in-band insertion loss, not requiring external matching components, and realizable in compact, monolithic form.